The answer to this question can be found in Chapter 3. This is when Gabriel comes home with his father for the first time, and Lily is the first character to notice that this new arrival is different from everybody else, like Jonas is. What makes this fact unsettling is explained in the following quote:

Almost every citizen in the community had dark eyes. His parents did, and Lily did, and so did all of his group members and friends. But there were a few exceptions...

What is clear about this group is that being the same and working together is a principle core value. Being marked out as different, as being separate from this group, even in a physical way, is something that Jonas obviously finds very difficult to cope with. In the culture of this future society, the narrator tells the reader that it was "considered rude to call attention to things that were unsettling or different about individuals." This is a society where differences are ignored and not mentioned for fear of shaming and embarrassment, rather than celebrated. This is why the similarity in the paleness of the eyes of Jonas and Gabriel is not mentioned. From the book, it can be seen that paleness of eyes seems to indicate some kind of special link between Jonas and Gabriel, which is later reinforced when Jonas acts to save Gabriel, running away from his community, and also when he shares his memories that he himself has received with Gabriel too.